Method for preparing acrylic acid

ABSTRACT

Provided is a process for preparing acrylic acid comprising (1) preparing acrolein by catalytic gas phase oxidation comprising (a) providing a reaction gas comprising (i) 5 to 10 mol % propylene, (ii) 0.02 to 0.75 mol % propane, and (iii) 0.25 to 1.9 mol % of a fuel mixture comprising at least one of methane and ethane, wherein the molar ratio of the total amount of propane, methane, and ethane to the total amount of propylene is from 0.01:1 to 0.25:1, (b) contacting the reaction gas with a first mixed metal oxide catalyst to form a mixture comprising acrolein, wherein the first mixed metal oxide catalyst comprises one or more of molybdenum, bismuth, cobalt, and iron, and (2) contacting the acrolein mixture with a second mixed metal oxide catalyst to form a mixture comprising acrylic acid, wherein the second mixed metal oxide catalyst comprises one or more of molybdenum, vanadium, tungsten, copper, and antimony.

FIELD OF THE INVENTION

This invention relates generally to a method for preparing acrylic acidby catalytic gas phase oxidation. The method includes providing areaction gas containing propylene, propane, and a fuel mixtures of atleast one of methane and ethane, and contacting it with a firstoxidation catalyst to form a mixture containing acrolein, and contactingthe acrolein mixture with a second oxidation catalyst to form a mixturecontaining acrylic acid.

BACKGROUND

Acrylic acid can be produced commercially by selective oxidation ofacrolein, which can be produced by selective oxidation of propylene.Commercially available propylene can be divided into different gradesbased on the levels of other impurities, e.g., refinery grade, chemicalgrade, and polymer grade. Depending on the price differential, there canbe an advantage to using one grade over the other. While differentgrades of propylene can be used as a feed in producing acrolein viacatalytic oxidation, changing the propylene grade from one to anothercan have a significant impact on the fuel content of the absorber offgas, and can also be prohibitive due to the fact that acrolein plantsare typically designed for a fixed propylene composition. For example,an existing plant designed for Chemical Grade Propylene (containing 3 to7% by volume propane as the main impurity) faces a few challenges withhigh purity Polymer Grade Propylene (containing less than 0.5% by volumepropane as the main impurity): (1) a shortage of propane fuel (animpurity in the propylene) that is used as a fuel for downstream thermaloxidation of volatile organics prior to venting into the atmosphere; and(2) a shortage of propane as a ballast gas that moves the reactor feedcomposition away from the flammable region.

Different propylene grades used as a feed gas for producing acroleinhave been utilized in the art. For example, WO 2014/195157 A1 disclosesa method of producing acrolein with feed gas containing Refinery GradePropylene and a specified range of sulfur and unsaturated hydrocarbons.The prior art does not, however, disclose a method for preparingacrolein via gas phase oxidation by providing a reaction gas accordingto the present invention which allows for the use of a high grade ofpropylene in the reactor feed gas for an existing plant designed forChemical Grade Propylene without sacrificing production rate orrequiring other capital improvements.

Accordingly, there is a need to develop a method that allows for the useof a reactor feed gas containing a high grade of propylene, while notsuffering from the drawbacks of the shortage of fuel for downstreamthermal oxidation of volatile organics, and shortage of a ballast gasthat moves the reactor feed composition away from the flammable region.

STATEMENT OF INVENTION

One aspect of the invention provides a process for preparing acrylicacid comprising (1) preparing acrolein by catalytic gas phase oxidationcomprising (a) providing a reaction gas comprising (i) 5 to 10 mol %propylene, (ii) 0.02 to 0.75 mol % propane, and (iii) 0.25 to 1.9 mol %of a fuel mixture comprising at least one of methane and ethane, whereinthe molar ratio of the total amount of propane, methane, and ethane tothe total amount of propylene is from 0.01:1 to 0.25:1, (b) contactingthe reaction gas with a first mixed metal oxide catalyst to form amixture comprising acrolein, wherein the first mixed metal oxidecatalyst comprises one or more of molybdenum, bismuth, cobalt, and iron,and (2) contacting the acrolein mixture with a second mixed metal oxidecatalyst to form a mixture comprising acrylic acid, wherein the secondmixed metal oxide catalyst comprises one or more of molybdenum,vanadium, tungsten, copper, and antimony.

Another aspect of the invention provides process for preparing acrylicacid comprising (1) preparing acrolein by catalytic gas phase oxidationcomprising (a) providing a reaction gas comprising (i) 7.5 to 8.2 mol %propylene, (ii) 0.03 to 0.62 mol % propane, and (iii) 0.5 to 1.4 mol %of a fuel mixture comprising at least one of methane and ethane, whereinthe fuel mixture comprises sulfur in an amount of less than 30 parts permillion by volume of the fuel mixture, wherein the molar ratio of thetotal amount of propane, methane, and ethane to the total amount ofpropylene is from 0.1:1 to 0.18:1, and (b) contacting the reaction gaswith a first mixed metal oxide catalyst to form a mixture comprisingacrolein, wherein the oxidation catalyst comprises a first mixed metaloxide catalyst comprising a primary component selected from the groupconsisting of molybdenum, bismuth, and combinations thereof, and asecondary component selected from the group consisting of cobalt, iron,nickel, zinc, tungsten, phosphorous, manganese, potassium, magnesium,silicon, aluminum, and combinations thereof, wherein the primarycomponent and secondary component are in an atomic ratio of from 9:28 to28:9, and (2) contacting the acrolein mixture with a second mixed metaloxide catalyst to form a mixture comprising acrylic acid, wherein thesecond mixed metal oxide catalyst comprises a primary component selectedfrom the group consisting of molybdenum, vanadium, and combinationsthereof, and a secondary component selected from the group consisting oftungsten, cobalt, copper, and combinations thereof, wherein the primarycomponent and secondary component are in an atomic ratio of from 1:1 to11:1.

DETAILED DESCRIPTION

The inventors have now surprisingly found that acrylic acid can beprepared from acrolein prepared by catalytic gas phase oxidation of areaction gas containing a high grade propylene while avoiding theshortage of fuel for downstream thermal oxidation of volatile organics,and the shortage of ballast gas that moves the reactor feed compositionaway from the flammable region. Such drawbacks are avoided by includinga fuel mixture comprising at least one of methane and ethane as asupplement to avoid the effects that would otherwise result from usinghigh grades of propylene containing relatively lower amounts of propaneas an impurity. Accordingly, the present invention provides in oneaspect a process for preparing acrylic acid comprising (1) preparingacrolein by catalytic gas phase oxidation comprising (a) providing areaction gas comprising (i) 5 to 10 mol % propylene, (ii) 0.02 to 0.75mol % propane, and (iii) 0.25 to 1.9 mol % of a fuel mixture comprisingat least one of methane and ethane, wherein the molar ratio of the totalamount of propane, methane, and ethane to the total amount of propyleneis from 0.01:1 to 0.25:1, (b) contacting the reaction gas with a firstmixed metal oxide catalyst to form a mixture comprising acrolein,wherein the first mixed metal oxide catalyst comprises one or more ofmolybdenum, bismuth, cobalt, and iron, and (2) contacting the acroleinmixture with a second mixed metal oxide catalyst, wherein the secondmixed metal oxide catalyst comprises one or more of molybdenum,vanadium, tungsten, copper, and antimony.

The inventive process comprises providing a reaction gas that iscontacted with an oxidation catalyst to form a mixture containingacrolein. The reaction gas contains propylene, propane, and a fuelmixture containing at least one of methane and ethane. The reaction gascontains propylene in an amount of from 5 to 10 mol %, preferably from6.5 to 9 mol %, and more preferably from 7.5 to 8.2 mol %, based on thetotal volume of the reaction gas. The reaction gas contains propane inan amount of from 0.02 to 0.75 mol %, preferably from 0.02 to 0.65 mol%, and more preferably from 0.03 to 0.62 mol %, based on the totalvolume of the reaction gas. The reaction gas contains a fuel mixturecontaining at least one of methane and ethane in an amount of from 0.25to 1.9 mol %, preferably from 0.4 to 1.6 mol %, and more preferably offrom 0.5 to 1.4, based on the total volume of the reaction gas. Incertain embodiments, the reaction gas contains methane in an amount offrom 0.5 to 1.9 mol %, preferably from 0.8 to 1.6 mol %, and morepreferably of from 1.1 to 1.4 mol %, based on the total volume of thereaction gas. In certain embodiments, the molar ratio of the totalamount of propane, methane, and ethane in the reaction gas to the totalamount of propylene in the reaction gas is from 0.1:1 to 0.25:1,preferably from 0.1:1 to 0.2:1, and more preferably from 0.1:1 to0.18:1.

The reaction gas further contains an oxidant for the oxidation ofpropylene to acrolein, and acrolein to acrylic acid. Suitable oxidantsinclude, for example, oxygen (O₂). Suitable sources of oxygen include,for example, air or a source that contains a higher purity of O₂. Incertain embodiments, the molar ratio of O₂ to propylene is in the rangeof from 1.6:2.2, preferably from 1.7:2.0.

The reaction gas of the inventive process is contacted with an oxidationcatalyst—a first mixed metal oxide catalyst. Mixed metal oxidescatalysts that are known in the art, e.g., as described in U.S. Pat.Nos. 6,028,220, 8,242,376, and 9,205,414. Suitable first mixed metaloxide catalysts include, for example, those including one more ofmolybdenum, bismuth, cobalt, iron, nickel, zinc, tungsten, phosphorous,manganese, potassium, magnesium, silicon, and aluminum. In certainembodiments, the first mixed metal oxide catalyst comprises one or moreof molybdenum, bismuth, cobalt, and iron. In certain embodiments, thefirst mixed metal oxide catalyst comprises primary and secondarycomponents in an atomic ratio of from 9:28 to 28:9, preferably from11:28 to 20:9, and more preferably of from 13:28 to 14:9. In certainembodiments, the primary component comprises one or more of molybdenumand bismuth. In certain embodiments, the secondary component comprisesone or more of cobalt, iron, nickel, zinc, tungsten, phosphorous,manganese, potassium, magnesium, silicon, and aluminum.

In certain embodiments, the fuel mixture contains methane that issourced from natural gas that includes impurities that are detrimentalto the oxidation catalyst, e.g., catalyst poisons such as various sulfurcompound (e.g., H₂S, dimethyl sulfide, carbonyl sulfide, mercaptans, andthe like). Gas containing such catalyst poisons are known in the art as“sour gas.” Sour gas can be “sweetened” by removing such sulfurcompounds from the natural gas. Sulfur compounds can be removed it theirpresence has negative impacts on the catalyst performance, or downstreamthermal oxidizer. Suitable sulfur removal technologies are known in theart and include, for example, by flowing the natural gas through a fixedbed packed with absorbent materials. In certain embodiments, the fuelmixture contains sulfur in an amount of less than 30 parts per millionby volume of the fuel mixture, preferably less than 5 parts per million,more preferably less than 1 part per million, and even more preferablyless than 0.1 part per million, by volume of the fuel mixture.

In certain embodiments, the inventive process step of contacting thereaction gas to form a mixture comprising acrolein comprises passing thereaction gas through a reactor tube or through a plurality of reactortubes in parallel, each of which is filled with the first mixed metaloxide catalyst. In certain embodiments, the one or more reactor tubesare charged with the first mixed metal oxide catalyst to a length offrom 1 to 7 meters, preferably from 2 to 6 meters, and more preferablyof from 3 to 5 meters. In certain embodiments, the internal diameter ofeach reactor tube is in the range of from 15 to 50 mm, preferably 20 to45 mm, and more preferably of from 22 to 40 mm

The preparation of acrylic acid further comprises contacting theacrolein mixture obtained by the inventive process described above witha mixture of oxidation catalysts—the first mixed metal oxide catalystdescribed above and a second mixed metal oxide catalyst to produce amixture containing acrylic acid. Suitable second mixed metal oxidecatalysts are known in the art, e.g., as described in U.S. Pat. Nos.4,892,856 and 6,762,148, and include, for example, one or more ofmolybdenum, vanadium, tungsten, copper, and antimony. In certainembodiments, the second mixed metal oxide catalyst comprises primary andsecondary components in an atomic ratio of from 1:1 to 11:1, preferablyfrom 2:1 to 9:1, and more preferably of from 3:1 to 7:1. In certainembodiments, the primary component comprises one or more of molybdenumand vanadium. In certain embodiments, the secondary component comprisesone or more of tungsten, copper, and antimony.

In certain embodiments, the inventive process step of contacting thereaction gas to form a mixture comprising acrolein comprises passing thereaction gas through a reactor tube or through a plurality of reactortubes in parallel, each of which is filled with a mixture of the firstmixed metal oxide catalyst and the second mixed metal oxide catalyst. Incertain embodiments, the one or more reactor tubes are charged withmixed metal oxide catalysts to a length of from 1 to 7 meters,preferably from 2 to 6 meters, and more preferably of from 3 to 5meters. In certain embodiments, the internal diameter of each reactortube is in the range of from 15 to 50 mm, preferably 20 to 45 mm, andmore preferably of from 22 to 40 mm.

Some embodiments of the invention will now be described in detail in thefollowing Examples.

EXAMPLES Example 1 Characterization of Thermal Oxidation Constraints onExemplary and Comparative Processes

A conventional two stage single pass acrylic acid process is operatedwith typical conditions on Chemical Grade Propylene (“CGP”), PolymerGrade Propylene (“PGP”), and PGP with supplemental fuel, as recited inTable 1.

TABLE 1 Thermal Oxidation Constraints on Exemplary and ComparativeProcesses Operation Operation Operation w/PGP + fuel w/CGP w/PGPinjection Relative C₃H₆ Rate 100 72 100 (% of maximum) C₃H₆ Purity (mol%) 94.50 99.50 99.50 C₃H₆ Concentration 8.0 8.0 8.0 (mol %) C₃H₈Concentration 0.47 0.04 0.05 (mol %) Supplemental C₁-C₃ 0.00 0.00 1.07Fuel Concentration (mol %) Total C₁-C₃ Fuel 0.47 0.04 1.11 Concentration(mol %) C₃H₆ Conversion 96.0 96.0 96.0 (%) Residual Acrolein 0.50 0.500.50 Yield (%) Total C₁-C₃ 0.058 0.005 0.139 fuel:propylene (mol ratio)AOG⁺ Heating 33 18 33 Value (Btu/SCF) Thermal Oxidizer 965 964 965Firebox Temperature (° C.) Stack O₂ (mol %) 2.95 2.94 2.94 Tox BurnerCapacity 100 100 100 (%) ⁺“AOG” represents the Absorber Off Gas

The results demonstrate that the process is constrained by energy inputto the thermal oxidizer. Operation with the above conditions results ina vapor waste stream containing 30 to 40% of the energy input to thethermal oxidizer. As the purity of the propylene feedstock increases,the energy content in the vapor waste stream decreases. At the extremecase of a polymer grade propylene feed with propylene 99.5% minimum, theenergy content in the vapor waste stream is 50% of what it was with CGP.Without any other process changes, the plant production capacity wouldhave to be decreased by 20-30% to maintain the desired thermal treatmentconditions.

To avoid the rate reduction from the energy input limitation, naturalgas (or C₁ to C₃ fuel) is injected into the process at a 0.14:1 molarratio with propylene. The energy no longer provided by the “impurities”in the propylene is replaced with energy from lower cost methane. Thisallows the plant to operate with a high purity feedstock whilemaintaining the operating rate, realizing a reduction in energy cost tooperate the thermal treatment unit, and avoiding capital modification ofthermal treatment unit.

Example 2 Characterization of Flammability Constraints on Exemplary andComparative Processes

One hazard inherent in the oxidation of propylene is the management ofhazards associated with the flammability of propylene. This hazard canbe managed by operating with a reactor feed composition outside of theflammable region by some margin of safety. The distance between theoperating point and the flammable region is defined as the approach tothe flammable limit. Margins of safety exist to cover error in flammableboundary correlations, errors in determination of reactor feedcomposition, and to prevent reactor trips associated with disturbancesin reactor feed flows. The reactor feeds are manipulated such that thefeed composition is moved above the upper flammable limit withoutpassing through the flammable region. When one is above the flammablelimit, increasing fuel content tends to increase the oxygen required tocreate a flammable mixture (more fuel increases the distance to theflammable limit). In a propylene partial oxidation process, propyleneconcentration cannot be increased independently because oxygen isrequired in a particular molar ratio (typically greater than 1.4:1) topropylene to complete the desired chemical reaction. Because of theoxygen to propylene constraint, the oxygen concentration must alsoincrease when C₃ concentration increases. The net result of increasingpropylene concentration at constant oxygen to propylene ratio is movingcloser to the flammable region. A conventional two stage acrylic acidprocess with Absorber Off Gas recycle is operated with typicalconditions, as recited in Table 2.

TABLE 2 Flammability Constraints on Exemplary and Comparative ProcessesOperation Operation Operation w/PGP + fuel w/CGP w/PGP injectionRelative C₃H₆ Rate 100 94.5 100 (% of maximum) C₃H₆ Purity (mol %) 94.5099.50 99.50 C₃H₆ Concentration 7.7 7.4 7.7 (mol %) C₃H₈ Concentration0.58 0.05 0.05 (mol %) Supplemental C₁-C₃ 0.00 0.00 1.28 FuelConcentration (mol %) Total C₁-C₃ Fuel 0.58 0.05 1.33 Concentration (mol%) C₃H₆ Conversion 96.5 96.5 96.5 (%) Residual Acrolein 1.00 1.00 1.00Yield (%) Total C₁-C₃ 0.075 0.007 0.173 fuel:propylene (mol ratio) AOGHeating 28 15 28 Value (Btu/SCF) Thermal Oxidizer 899 899 898 FireboxTemperature (° C.) Stack O₂ (mol %) 2.1 2.1 2.1 Tox Burner Capacity 70100 70 (%) Approach to minimum minimum >minimum Flammable LimitCompressor 100 100 100 Capacity (%)

The results demonstrate that the process is simultaneously constrainedby the ability of the compressor to pump mixed gas (air+recycle) to thereactor, the propylene concentration in the reactor feed, and the oxygenin the reactor outlet. Each one of these constraints represents asignificant boundary. In turn, it is not possible to increase thecapacity of a compressor without making capital investment. Operatingtoo closely to the flammable region risks a process disturbance thatcould cause fire with significant safety and economic impact. Ifadequate excess oxygen is not maintained in the reactor outlet, it maycause the catalyst to age prematurely or cause incomplete conversion ofacrolein to acrylic acid and high levels of acrolein fed to the thermaloxidizer. Insufficient oxygen could potentially cause a release ofacrolein if the thermal oxidizer is unable to handle the increasedacrolein loading. Thus, in a process constrained as defined above andoperated at the conditions defined above, the maximum operating rate hasto be reduced by 5% (on propylene basis) when the propylene purityincreases. By injecting natural gas into the propylene at 0.17:1 C1 toC₃:C₃H₆ molar ratio, the maximum rate can be maintained with higherpurity of propylene. In addition, the “approach to flammability limit”is moved further away from the flammable region. The advantage of havingan inert fuel present in the reactor is regained.

What is claimed is:
 1. A process for preparing acrylic acid comprising:(1) preparing acrolein by catalytic gas phase oxidation comprising (a)providing a reaction gas comprising (i) 5 to 10 mol % propylene, (ii)0.02 to 0.75 mol % propane, and (iii) 0.25 to 1.9 mol % of a fuelmixture comprising at least one of methane and ethane, wherein the molarratio of the total amount of propane, methane, and ethane to the totalamount of propylene is from 0.01:1 to 0.25:1; (b) contacting thereaction gas with a first mixed metal oxide catalyst to form a mixturecomprising acrolein, wherein the first mixed metal oxide catalystcomprises one or more of molybdenum, bismuth, cobalt, and iron; and (2)contacting the acrolein mixture with a second mixed metal oxide catalystto form a mixture comprising acrylic acid, wherein the second mixedmetal oxide catalyst comprises one or more of molybdenum, vanadium,tungsten, copper, and antimony.
 2. The process of claim 1, wherein thefuel mixture comprises methane.
 3. The process of claim 1, wherein thereaction gas comprises propylene in an amount of from 7.5 to 8.2 mol %.4. The process of claim 1, wherein the reaction gas comprises propane inan amount of from 0.03 to 0.62 mol %.
 5. The process of claim 1, whereinthe reaction gas comprises the fuel mixture in an amount of from 0.5 to1.4 mol %.
 6. The process of claim 2, wherein the reaction gas comprisesmethane in an amount of from 1.1 to 1.4 mol %.
 7. The process of claim1, wherein the molar ratio of the total amount of propane, methane, andethane to the total amount of propylene is from 0.1:1 to 0.18:1.
 8. Theprocess of claim 1, wherein the first mixed metal oxide catalystcomprises a primary component selected from the group consisting ofmolybdenum, bismuth, and combinations thereof, and a secondary componentselected from the group consisting of cobalt, iron, nickel, zinc,tungsten, phosphorous, manganese, potassium, magnesium, silicon,aluminum, and combinations thereof, wherein the primary component andsecondary component are in an atomic ratio of from 9:28 to 28:9, andwherein the second mixed metal oxide catalyst comprises a primarycomponent selected from the group consisting of molybdenum, vanadium,and combinations thereof, and a secondary component selected from thegroup consisting of tungsten, cobalt, copper, and combinations thereof,wherein the primary component and secondary component are in an atomicratio of from 1:1 to 11:1.
 9. The process of claim 1, wherein the fuelmixture comprises sulfur in an amount of less than 30 parts per millionby volume of the fuel mixture.
 10. A process for preparing acrylic acidcomprising: (1) preparing acrolein by catalytic gas phase oxidationcomprising (a) providing a reaction gas comprising (i) 7.5 to 8.2 mol %propylene, (ii) 0.03 to 0.62 mol % propane, and (iii) 0.5 to 1.4 mol %of a fuel mixture comprising at least one of methane and ethane, whereinthe fuel mixture comprises sulfur in an amount of less than 30 parts permillion by volume of the fuel mixture, wherein the molar ratio of thetotal amount of propane, methane, and ethane to the total amount ofpropylene is from 0.1:1 to 0.18:1; and (b) contacting the reaction gaswith a first mixed metal oxide catalyst to form a mixture comprisingacrolein, wherein the first mixed metal oxide catalyst comprises aprimary component selected from the group consisting of molybdenum,bismuth, and combinations thereof, and a secondary component selectedfrom the group consisting of cobalt, iron, nickel, zinc, tungsten,phosphorous, manganese, potassium, magnesium, silicon, aluminum, andcombinations thereof, wherein the primary component and secondarycomponent are in an atomic ratio of from 9:28 to 28:9; and (2)contacting the acrolein mixture with a second mixed metal oxide catalystto form a mixture comprising acrylic acid, wherein the second mixedmetal oxide catalyst comprises a primary component selected from thegroup consisting of molybdenum, vanadium, and combinations thereof, anda secondary component selected from the group consisting of tungsten,cobalt, copper, and combinations thereof, wherein the primary componentand secondary component are in an atomic ratio of from 1:1 to 11:1.